The present invention relates to a method for controlling a converter according to the preamble to the accompanying claim 1 and to a converter according to the preamble to the accompanying independent converter claim.
The invention thus relates to conversion of voltage in which at least one side of the converter carries a dc voltage, but the other side does not necessarily have to carry an ac voltage, but said output may also belong to a second side of a converter in the form of a dc/dc converter used for changing the level of a dc voltage. However, for the purpose of elucidating the invention, but consequently not limiting the same, the case of a converter with said output connected to an ac voltage line, that is, conversion between dc voltage and ac voltage, will henceforth be described. To this end, the case of a converter of VSC (Voltage Source Converter) type intended to generate a train of pulses, by switching between said main states, with a definite amplitude according to a pulse-width modulation pattern on the output of the converter will be described. Such a converter may be used in all situations where dc voltage is to be converted into ac voltage and vice versa, whereby examples of such uses are in stations of high-voltage direct current (HVDC) installations, in which the dc voltage is normally converted into three-phase ac voltage or vice versa, or in so-called back-to-back stations where ac voltage is first converted into dc voltage and this dc voltage then converted into ac voltage, as well as in SVCs (Static Var Compensators), where the dc-voltage side consists of one or more freely hanging capacitors. The ac side of the converter could also be connected to an ac motor for driving it, or to an ac generator.
Further, it is pointed out that the method is directed to control of a said converter that exhibits at least said six units, which means that at least three different levels may be obtained for the voltage on said output, but it is fully possible for the converter to exhibit several such units, so that more than four main states and also more than three levels of the voltage on the output may be achieved. In this context, several converters of this kind may form part of a converter for several phases, such as for three phases, but it may also be designed to form on its own a converter for conversion between dc voltage a single-phase ac voltage.
Further, the invention is not limited to any special voltage levels of said first dc voltage side or magnitude of power that is to be handled. The former is advantageously within the interval of 1 kV to 500 kV.
One advantage of using a converter with at least three levels instead of a two-level bridge when converting ac voltage to dc voltage is that considerably lower frequencies for switching the semiconductor elements of the units according to the pulse-width modulation pattern may be used for achieving a curve shape of the ac voltage side of a given quality. In this way, the switching losses may be considerably reduced so that it is also possible to transmit higher powers through such a three-level converter than through a two-level bridge, since higher on-state losses may be allowed. At the same time, harmonics generated by the pulse-width modulation process are reduced.
One method of the kind defined in the introductory part of the description is previously known from applicant's own Swedish patent 517 427. This Swedish patent describes a method that constitutes an improvement of prior art such methods for control of a converter with said six units by proposing how the switching losses are to be distributed more uniformly than previously between the different units. By utilizing, in an embodiment of the method according to Swedish patent 517 427, only four different states of the semiconductor elements of the units, the actual method for controlling the semiconductor elements will be very simple. It is pointed out that, in practice, there is, of course, a fifth possible state of this embodiment, namely when the converter is out of operation and when all the semiconductor elements are turned off. Since the semiconductor elements of the first and sixth units are controlled to assume the same position, turned on or off, in the respective main state, and since the semiconductor elements in the fourth and fifth units are controlled to assume the same position, turned on or off, in the respective main state, it is possible to use the same control signal for the semiconductor elements in the first and sixth units and in the fourth and fifth units, respectively.
Although it is advantageous to interlock, so to speak, the semiconductor elements in four of the units pair by pair in this way, the inventors of the present invention have realized that there may sometimes arise problems in giving the semiconductor elements of such a pair of units control signals for turning them on or off simultaneously. This is due to the fact that the semiconductor element will react differently to such a control signal in dependence on whether the semiconductor element is current-carrying during the switching or not, that is to say, whether it is a question of a passive voltage switching or an actual current commutation. A passive voltage switching may proceed considerably faster than an actual current commutation, so that in one case the semiconductor element is turned on or off significantly more rapidly than in the other case. Considering the condition of current direction during commutation, this may imply that brief high voltage peaks could be achieved across any said unit, which could destroy the semiconductor element in question. Alternatively, the semiconductor elements, or at least such elements in certain units, must be designed in most cases to be oversized as regards voltage withstand capability in order to manage such voltage peaks, which makes them unnecessarily expensive.